The source of irregularity in El Nino, the large interannual climate variation of the Pacific ocean-atmosphere system, has remained elusive. Results from an El Nino model exhibit transition to chaos through a series of frequency-locked steps created by nonlinear resonance with the Earth’s annual cycle. The overlapping of these resonances leads to the chaotic behavior. This transition scenario explains a number of climate model results and produces spectral characteristics consistent with currently available data.